The performance of an optical disc drive is greatly related to the optical quality of the inner system and spherical aberration is one of important factors for degrading the optical quality of the inner system. When the inner optical quality of the optical disc drive is affected with spherical aberration, the focus error signal and the tracking error signal generated by the optical disc drive are easily distorted so that the servo control becomes more difficult and the unfocused condition or off-track condition possibly happens. Moreover, the deformed light spot of the read/write laser light of the optical disc drive resulted from spherical aberration causes more errors in radio frequency signal when reading discs and poor write quality of the optical disc drive.
Please refer to FIG. 1, which is a diagram showing spherical aberration. The main reason why spherical aberration occurs is that an optical path difference Δλ between the marginal rays and the paraxial rays of the laser light exists when the marginal rays and the paraxial rays reaches the foci plane. The light spot at the foci plane becomes elliptic which resulted from spherical aberration. However, the shape of an optical light spot should be round. In addition the energy distribution of the laser light at the foci plane deteriorates because of spherical aberration which results in the affection of the read/write quality of the optical disc drive.
Generally there are two ways to compensate spherical aberration (SA) in an optical system. One is to adopt a liquid crystal SA compensator and the other is to adopt a collimating SA compensator.
The liquid crystal SA compensator is usually composed of liquid crystal material. The refractive index of the liquid crystal SA compensator is varied with the inputted voltage level, thus the laser light which passes through the liquid crystal SA compensator is compensated so that the shape of the light spot which the laser light reaches the optical disc is circular. The second way to compensate spherical aberration is to adopt a collimating SA compensator. The collimating SA compensator is composed of collimating lens. The position of the collimating lens is changed so as to adjust the optical path difference between the marginal rays and the paraxial rays of the laser light so that spherical aberration is compensated.
That is to say, the optical path difference Δλ between the marginal rays and the paraxial rays of the laser light reaching to the foci plane is compensated once the SA compensating value is inputted to the SA compensator.
Please refer to FIG. 2A, FIG. 2B and FIG. 2C, which illustrates simulated spot diagrams of the optical disc drive. As shown in FIG. 2B, the optimal focus point is at position “0” when a cover layer of an optical disc is 0.6 mm and the SA compensating value “0” is provided. When the thickness of the optical disc (DVD) varies from 0.6 mm to 0.55 mm due to process variation, the optimal focus point shifts 50 μm to the negative direction as shown in FIG. 2A, while the thickness of the optical disc (DVD) varies from 0.6 mm to 0.65 mm due to process variation, the optimal focus point shifts 50 μm to the positive direction as shown in FIG. 2A. However, the quality of the optical signal measured in the optimal focus point of FIG. 2A or FIG. 2C is already deteriorated which result in that the spot size in the optimal focus point of FIG. 2A or FIG. 2C is rather bigger than the spot size in the optimal focus point of FIG. 2B.
In other words, the optimal focus point shifts when a spherical aberration exists in the optical system, the corresponding optimal SA compensating value at the shifted focus point is to be acquired so that the spherical aberration can be compensated. Likewise, when the SA compensating value or the optical path difference of the laser light is adjusted which compensates the spherical aberration, the optimal focus point of the optical disc drive would also shifts. Therefore the best way to assure the best read/write quality of the optical disc drive (i.e. assure the optical disc drive positioned at the optimal focus point) is to adjust the variables of focus bias FEbias and SA compensating value SAvalue at the same time.
However, most prior arts only provide methods for determining an optimal SA compensating value. Please refer to FIG. 3, which illustrates a method for adjusting SA compensating value in U.S. Publication 2008/0074973. Firstly, an amplitude of an tracking error signal TEVPP is adopted as an index signal for quantifying spherical aberration. The amplitudes of the tracking error signal corresponding to SA compensating values (SA=A, SA=0, SA=−A) are measured. The optimal SA compensating value SA_peak corresponding to the largest amplitude of the tracking error signal is then acquired by performing second-order approximation of the three measured point. However, if the optimal SA compensating value SA_peak is not within a limited range, the above steps are repeated until the optimal SA compensating value SA_peak within the limited range is acquired. Nevertheless, the above method for adjusting SA compensating value omits the fact of the optimal focus point shifting. Even the spherical aberration is corrected, the focus point corresponding the adjusted SA compensating value is not at the optimal position.
Please refer to FIG. 4, which illustrates an effectiveness diagram of the SA compensating value and the focus bias versus spherical aberration. An index signal for quantifying the spherical aberration is adopted, i.e. an amplitude of reproduction jitter for example. The index signal positioned more inside the ellipse represents that the value of the index signal is close to an extreme value and the image quality of the laser light is optimal. If the optical disc drive sets an initial focus point FBinitial at a deviated area as shown in FIG. 4, the optimal SA compensating value SAoptimal corresponding to the initial focus bias is actually not the optimal SA compensating value of the optical disc drive. The optical disc drive with the optimal SA compensating value SAoptimal does not have best read/write quality because spherical aberration still occurs. Therefore, the focus bias FEbias and the SA compensating value SAvalue are to be adjusted at the same time which ensures the optical disc drive having the best read/write quality.
Hence, U.S. Pat. No. 7,344,077 provides a method for adjusting the focus bias and the SA compensating values at the same time. The method mainly searches different combinations of focus bias and SA compensating value in 2D direction and a quadratic curve approximation is performed on the searched points so that an optimal combination of focus bias and SA compensating value is acquired. The method finds the most optimal combination of focus bias and SA compensating value, however the quadratic curve approximation is too complex and wasting a lot of system resource and memory space. Therefore, How to acquire an optimal combination of focus bias and SA compensating value rapidly and efficiently is the subject matter of the present invention.